I'm trying to figure out the cause for these diagnostic codes. I've replaced the MAF, air, and fuel filters. My fuel pressure read 30 psi at idle, 38 with the vacuum hose removed from the fuel pressure regulator, and 38 at WOT. I got the same readings using both tanks. Although within specs this seems a little low to me. Could this cause the codes? I would have expected pressure to go closer to 45 PSI when I disconnected the fuel pressure regulator.

Per other posts I'm going to look for vacuum leaks as well.

Thanks for the help!

1996 F350 7.5 EOD. all stock.

 

I take it this is a California Truck.
Yes either the fuel pressure gauge reads low (check it on another truck or car) or the FPR has a weak spring.
If it is low the filter may need to be changed or the pump is weak.

 

Well this is getting reeeallllllyyyy frustrating.
The truck is a California model. I am unable to find anything with the fuel system or a vacuum leak.
I've inspected all the vacuum hoses, had 2 shops look for leaks, including the 'smoke test' with no luck. I even took the air intake and all hoses off for inspection.
I've replaced the following:
MAF sensor, Bank 1 & 2 O2 sensors (but not the CAT sensor), Fuel pressure regulator, fuel injectors (lots of cracked o-rings), fuel filter, and fuel pumps (2 tank system).

Fuel pressure went up a couple PSI with the new pumps. The last mechanic that looked but could not find the problem said their pressure test was at 33 psi when mine showed 30, so I think my fuel pressure is OK. This mechanic also admitted they would recommend a new PCM and intake manifold gasket replacement as options :-(

The only things I know to be wrong with the truck are:
The AC Compressor is failing and makes 'funny noises'
The transmission needs to be re-sealed.

Help!

 

Check for air leak downstream of maf. Also check exhaust leaks. If you have access to a shop with wideband o2 and datalog, you could have them datalog and check afr's to see if they can narrow down what is causing the lean code to trigger hego problem. Your fuel pressure readings are fine. Are these the only codes you are getting?

 

73FOMO,
Thanks for the input. I have looked for exhaust and and leaks between the MAF and intake manifold. On he 96 F_350 I have the ODB II set up and those codes mean lean fuel vs. HEGO. I'm not sure what practical difference that is.

Good point on the wideband/datalog approach. I'll see if I can find one that won't charge more than the truck is worth.

Another thing I forgot to add, is that the code is easiest to generate by warming up the truck, then in neutral rev to 3K RPM for few seconds, then rev hard to 4500RPM, then let it fall to idle. Do that a few times and the codes get set.

 

So I finally broke down and decided to re-gasket the intake manifold as I can't find any leaks elsewhere. I got the upper intake manifold off and viola! Oil on the manifold under the gasket. There's a big pool on the front left that extends under the gasket to the edge of the manifold but does not run out. I'm trying to decide whether to just re-gasket this part or go all the way and do the lower intake manifold as well. Thoughts?

I just uploaded pics of it to an album titled My Leaky Intake Manifold

Thanks
-jeff-

 

I can change upppers in 20 mins. so I would change it and see fixes the issue. If not, you only wasted 20 mins.

 

I have the same type of problem on my CA F250 460. Nagging P0171 and P0174. Did you ever resolve yours ? If so, was it the intake manifold? Thanks for any input !
Jerry

 

9 times out of 10 caused by a vacuum leak.

 

change your upper, then check for a leak, spray with water or brake cleaner at the gaps where the gasket is, as well as any vacuum lines and see if it runs different as you do so. I know some of the 5.0's in the mustangs had a weird grommet on the back side of the lower intake that would degrade over time, and you could only see it with the upper intake off, not sure if these versions have this, but check, also pcv valve stuck open?

 

Did this fix the problem? I am having all the same problems, and have done all of the same things accept the gasket.